It is well known that as the expansion ratio of an internal combustion engine is increased, more energy is extracted from the combustion gases and converted to kinetic energy and the thermodynamic efficiency of the engine increases. It is further understood that increasing air charge density increases both power and fuel economy due to further thermodynamic improvements. The objectives for an efficient engine are to provide a high-density charge, begin combustion at maximum density and then expand the gases as far as possible against a piston.
Conventional engines have the same compression and expansion ratios, the former being limited in spark-ignited engines by the octane rating of the fuel used. Furthermore, since in these engines the exploded gases can be expanded only to the extent of the compression ratio of the engine, there is generally substantial heat and pressure in the exploding cylinder which is dumped into the atmosphere at the time the exhaust valve opens resulting in a waste of energy and producing unnecessarily high polluting emissions.
Many attempts have been made to reduce the compression ratio and to extend the expansion process in internal combustion engines to increase their thermodynamic efficiency, the most notable one being the "Miller" Cycle engine, developed in 1947.
Unlike a conventional 4-stroke cycle engine, where the compression ratio equals the expansion ratio in any given combustion cycle, the Miller Cycle engine is a variant, in that the parity is altered intentionally. The Miller Cycle uses an ancillary compressor to supply an air charge, introducing the charge on the intake stroke of the piston and then closing the intake valve before the piston reaches the end of the inlet stroke. From this point the gases in the cylinder are expanded to the maximum cylinder volume and then compressed from that point as in the normal cycle. The compression ratio is then established by the volume of the cylinder at the point that the inlet valve closed, being divided by the volume of the combustion chamber. On the compression stroke, no actual compression starts until the piston reaches the point the intake valve closed during the intake stroke, thus producing a lower-than-normal compression ratio. The expansion ratio is calculated by dividing the swept volume of the cylinder by the volume of the combustion chamber, resulting in a more-complete-expansion, since the expansion ratio is greater than the compression ratio of the engine.
In the 2-stroke engine the Miller Cycle holds the exhaust valve open through the first 20% or so of the compression stroke in order to reduce the compression ratio of the engine. In this case the expansion ratio is probably still lower than the compression ratio since the expansion ratio is never as large as the compression ratio in conventional 2-stroke engines.
The advantage of this cycle is the possibility of obtaining an efficiency higher than could be obtained with an expansion ratio equal to the compression ratio. The disadvantage is that the Miller Cycle has a mean effective pressure lower than the conventional arrangement with the same maximum pressure, but with no appreciable improvements in emissions characteristics.
The Miller Cycle is practical for engines that are not frequently operated at light-loads, because at light-load operation the mean cylinder pressure during the expansion stroke tends to be near to, or even lower than, the friction mean pressure. Under such circumstances the more-complete-expansion portion of the cycle may involve a net loss rather than a gain in efficiency.
This type of engine may be used to advantage where maximum cylinder pressure is limited by detonation or stress considerations and where a sacrifice of specific output is permissible in order to achieve the best possible fuel economy. The cycle is suitable only for engines that operate most of the time under conditions of high mechanical efficiency, that is, at relatively low piston speeds and near full load.